This invention relates generally to electrostatic spray devices, and more particularly to an improved electrospray ion source assembly.
The electrospray (ES) process generally includes flowing a sample liquid into an electrospray ion source comprising a small tube or capillary which is maintained at a high voltage, in absolute value terms, with respect to a nearby surface. Conventional ES systems for mass spectrometry apply high voltage (relative to a ground reference) to the emitter electrode while holding the counter electrode at a lower, near ground reference voltage. For the positive ion mode of operation, the voltage on the emitter is high positive, while for negative ion mode the emitter voltage is high negative.
However, the emitter electrode can be held at (or near) the ground voltage. In this alternate configuration, the counter electrode is held at high negative voltage for positive ion mode and at high positive potential for negative mode. The voltage drop is the same between the electrodes and the electron flow in the circuit is the same in both the conventional and alternate bias configurations.
The liquid introduced into the tube or capillary is dispersed and emitted as fine electrically charged droplets (plume) by the applied electrical field generated between the tube or capillary which is held at high voltage, referred to as the working electrode, and the nearby surface. The nearby (e.g. 1 cm) surface is commonly referred to as the counter electrode.
The ionization mechanism generally involves the desorption at atmospheric pressure of ions from the fine electrically charged particles. The ions created by the electrospray process can then be used for a variety of applications, such as mass analyzed in a mass spectrometer.
The electrospray ion source operates electrolytically in a fashion analogous to a two-electrode controlled current (CCE) flow cell, effectively forming an electrochemical cell in a series circuit. A metal capillary or other conductive contact (usually stainless steel) placed at or near the point from which the charged ES droplet plume is generated (the ES emitter) is the working electrode in the system. The analytically significant reactions (in terms of ES-mass spectrometry (MS)) generally occur at this electrode.
The rate of charged droplet production by the electrospray source defines the average current (droplet generation rate times average charge per droplet) that flows in the cell (i.e., the ES current, iES). This rate is determined by several interactive variable parameters including the magnitude of the electric field applied between the working and counter electrodes, the solution flow rate, the solution viscosity and electrical conductivity. When used as an ion source for mass spectrometry, the counter electrode of the circuit is generally the atmospheric sampling aperture plate or inlet capillary, the various lens elements and detector of the mass spectrometer.
In a typical ES-MS process, a solution containing analytes of interest is pumped through the ES emitter which is held at high voltage, resulting in a charged solvent droplet spray or plume. The droplets drift towards the counter electrode under the influence of the electric field. As the droplets travel, gas-phase ions are liberated from the droplets. This process produces a quasi-continuous steady-state current with the charged droplets and ions constituting the current and completing the series circuit.
To sustain the buildup of an excess net charge on the surface of the liquid exiting the emitter, heterogeneous (electrode-solution) electron transfer reactions (i.e., electrochemical reactions) must occur at the conductive contact to the solution at the spray end of the ES device. Accordingly, oxidation reactions in positive ion mode (positive high voltage potentials) and reduction reactions in negative ion mode (negative high voltage potentials) will dominate at the ES emitter electrode. Electron transfer reactions also must occur at the counter electrode. Charge can flow in no other way than through these electrode circuit junctions. Thus, electrochemical reactions are inherent to the basic operation of the electrostatic sprayer used in ES applications, such as ES-MS.
The electrolysis reactions that take place in the ES emitter can influence the gas-phase ions formed and ultimately analyzed by the mass spectrometer, because they may change the composition of the solution from the composition that initially enters the ion source. These changes include, but are not limited to, analyte electrolysis resulting in ionization of neutral analytes or modification in the mass or charge of the original analyte present in solution, changes in solution pH through electrolytic H+ or OHxe2x88x92 production/elimination, and the introduction/elimination of specific species to/from solution (e.g., introduction of Fe2+ ions from corrosion of a stainless steel emitter).
Other than direct electrolysis of a particular species, redox chemistry or other chemistry can take place via homogenous solution reactions with a species that may be created at the working electrode. Homogeneous solution reactions are also used in controlled-current coulometry.
Applied to electrospray, a homogeneous solution reaction can occur though creating a species at the working electrode, and then diffusing the created species into solution and interacting it with another species causing an effect. This is a homogenous solution reaction, whereas reaction at the working electrode is heterogenous process. Homogeneous solution reactions provide the ability to greatly increase reaction efficiency because not all the analyte needs to get to the working electrode surface to react.
Sufficient time must generally be provided for the homogenous reaction to take place before the material is sprayed. Time between electrochemical reaction and spraying can be provided by an upstream working electrode contact. The electrochemical creation of reactants for the homogenous solution reaction can also buffer the potential to a given level, provided the species reacting is in high enough concentration or the reaction is not diffusion limited. A particular advantage of this approach is the ability to generate unstable reactants (e.g., the oxidant bromine) in situ.
Determining the extent and nature of these solution compositional changes is a complex problem. Because the magnitude of iES is known to be only weakly dependent on solvent flow rate, the extent of any solution compositional change that the electrolytic reactions can impose necessarily increases as flow rate decreases. The interfacial potential distribution of the working electrode ultimately determines what reactions in the system are possible as well as the rates at which they may occur.
However, in an ES ion source, the interfacial potential is not fixed, but rather adjusts to a given level depending upon a number of interactive variables to provide the required current to the circuit. The variables that are expected to materially affect the interfacial electrode potential include, but are not limited to, the magnitude of iES, the redox character and concentrations of all species in the system, the solution flow rate, the electrode material and geometry. Control over the electrochemical operation of the ES ion source is essential both to avoid possible analytical pitfalls it can cause (e.g. changes to the sample to be analyzed) and to fully exploit the phenomenon for new fundamental and analytical applications which are available through use of ES-MS.
Currently available electrospray emitter designs have not considered structures which can permit improved control of the electrochemistry of the electrochemical cell which can be used for analytical benefit. For example, current electrospray emitter designs do not perform efficient mass transport to the working electrode surface.
An electrospray device includes a high voltage electrode chamber having an inlet for receiving a fluid to be ionized and for directing fluid into the chamber and an outlet for transmitting fluid out from the chamber. At least one working electrode has an exposed surface within the chamber, the electrode for electrolytically producing ions from the fluid. A flow channel directs fluid in a flow direction over the surface of the electrode, a length of the flow channel over the electrode in the flow direction being greater than a height of the fluid flowing over the electrode. The electrospray device can include an emitter connected to the outlet for receiving the fluid from the outlet, the emitter for emitting a plume of gas phase ions.
An auxiliary electrode remotely located from the chamber can be provided for emission of ions generated by the working electrode toward the auxiliary electrode, the emission under influence of an electrical field between the electrodes. The emitter can include a non-electrically conductive capillary. A nebulizer can also be optionally added to the emitter to increase gas phase ion production.
The flow channel can include at least one capping member disposed on the working electrode. The capping member can define dimensions of the flow channel and is preferably formed from at least one chemically resistant polymer material. The capping member can include at least one electrode.
At least one dimension of the flow channel is preferably modifiable. The electrospray device can include a feedback and control system, the feedback and control system for modifying at least one channel dimension based on at least one measurement derived from the fluid transmitted from the electrode chamber.
The ratio of length of the flow channel over the electrode in the flow direction to the height of the fluid over the electrode can be at least 10, or preferably at least 100. More preferably, the ratio is at least 1000. Having the channel length over the working electrode greater than the height of the channel over electrode permits the electrospray device to substantially ionize or otherwise react substantially all analyte fluid flowing over the working electrode while maintaining a reasonable flow rate. The thin-layer fluid flow channel also minimizes the mass transport distance for reacting species in the fluid to reach the working electrode.
The working electrode can be disposed in an electrode support member. The electrode support can include at least two working electrodes. Different electrodes can be held at different electrical potentials. When multiple working electrodes are used in the electrode support, the respective electrodes can be formed from different materials, the different materials having different electrochemical potentials, different kinetic properties or different catalytic properties. A structure for application of the different potentials to the respective electrodes can be provided.
When working electrodes are provided in both the electrode support and capping member, the electrode support can be formed from a first material and the electrode in the capping member can be formed from a second material, the materials having different electrochemical potentials, different kinetic properties or different catalytic properties. In this configuration, a structure for applying a potential difference between the electrode in the electrode support and the electrode in the capping member is preferably provided. A voltage divider can be provided for application of a potential difference between working electrodes. When at least two working electrodes are provided, a switching network for switching connection to a high voltage power supply between respective electrodes is also preferably provided.
The surface of electrodes, the electrode support and the capping member can all be substantially planar. A flow member can be disposed between the capping member and the electrode support. In this configuration, the capping member can include at least one electrode.
An electrospray device includes a substantially planar high voltage electrode support including at least one working electrode having an exposed surface for electrolytically producing ions from fluid passing over the electrode, the working electrode support forming a bottom of a fluid flow channel. A capping member forms a top of the flow channel, the flow channel for directing the fluid in a flow direction over a surface of the electrode, a length of the flow channel over the electrode in the flow direction being greater than a height of the fluid flowing over the electrode. The capping member can include at least one electrode.
A mass spectrometer includes a high voltage electrode chamber having an inlet for receiving a fluid to be ionized and for directing the fluid into the chamber and an outlet for transmitting the fluid out from the chamber, at least one electrode having an exposed surface within the chamber, the electrode for electrolytically producing ions from the fluid. A flow channel directs the fluid in a flow direction over the surface of the electrode, a length of the flow channel over the electrode in the flow direction being greater than a height of the fluid flowing over the electrode. An orifice plate is remotely located from the chamber for receiving gas phase ions emitted from the emitter under influence of an electrical field between the electrode and orifice plate.
An electrochemical cell includes a high voltage electrode chamber having an inlet for receiving a fluid to be ionized and for directing the fluid into the chamber and an outlet for transmitting the fluid out from the chamber, and at least one electrode having an exposed surface within the chamber, the electrode for electrolytically producing ions from the fluid. A flow channel directs the fluid in a flow direction over the surface of the electrode, a length of the flow channel over the electrode in the flow direction being greater than a height of the fluid flowing over the electrode. A counter electrode is disposed remotely from the electrode chamber. The electrochemical cell can include a reference electrode in the electrode chamber.
A method of creating charged droplets includes the steps of providing a high voltage electrode chamber including an inlet for receiving a fluid to be ionized and for directing the fluid into the chamber and an outlet for transmitting the fluid out from the chamber and at least one working electrode having an exposed surface within the chamber, the electrode for electrolytically producing ions from the fluid. A flow channel directs the fluid in a flow direction over the surface of the working electrode, a length of the flow channel over the electrode in the flow direction being greater than a height of the fluid flowing over the electrode. The fluid is flowed into the electrode chamber. The length the fluid travels over the working electrode in the flow direction is greater than the height of the fluid over the working electrode. The method can include the step of emitting a plume of gas phase ions from ions generated by the working electrode. At least two electrodes can be provided in the chamber, the method including the step of dynamically switching an electrical potential between respective electrodes. When two or more electrodes are provided in the electrode chamber, the method can include the step of applying a potential difference between respective electrodes.
The method can include the step of dynamically changing at least one dimension of the flow channel. The channel height can preferably be dynamically changed. The dynamic changing can be responsive to at least one measured parameter relating to the fluid, the measured parameter being derived from the fluid. The dynamic changing step can include altering a force applied to the electrode chamber to modify the channel height. The plume of gas phase ions can be used for many processes. For example, the plume can be used for ion mobility spectrometry, spot preparation for matrix-assisted laser desorption mass spectrometry, crop dusting, paint spraying, ink jet printers, ink jet spotters, surface preparation of thin films and mass spectrometry.